Only set undo system if fun. exists.

The PR fixes an issue where spacemacs is in a bad state after upgrading. I
suppose what happens is that `evil-set-undo-system` is invoked before `evil` is necessarily the correct version.

CHANGELOG.develop

@@ -1365,6 +1365,7 @@ Other:
   - Fixed origami bindings in normal mode (thanks to Tomasz Kowal)
   - Handled buffer move/select/swap to last window nr +1 (thanks to duianto)
   - Set =evil-undo-system= to =undo-tree= (thanks to duianto)
+  - Checked that =evil-undo-system= exists before it's called (thanks to khjcph)
 *** Layer changes and fixes
 **** Agda
 - Fixes

layers/+distributions/spacemacs-bootstrap/packages.el

@@ -67,7 +67,8 @@
   (require 'evil)
   (evil-mode 1)
 
-  (evil-set-undo-system 'undo-tree)
+  (when (fboundp 'evil-set-undo-system)
+    (evil-set-undo-system 'undo-tree))
 
   ;; Use evil as a default jump handler
   (add-to-list 'spacemacs-default-jump-handlers 'evil-goto-definition)